The present invention relates to a polychrome matrix screen without coupling between the control rows and columns. It is used in optoelectronics in the construction of liquid crystal polychrome displays used as means for converting electrical information into optical information.
In per se known manner monochrome and polychrome matrix screens comprise a display cell constituted by two transparent insulating walls and a material having a plurality of zones distributed in matrix-like manner and inserted between a first group of electrodes covering one of the two walls and defining p control rows and a second group of electrodes covering the other wall, constituted by parallel conductive strips defining q control columns row i in which i is an integer such that 1.ltoreq.i--p and the column j in which j is an integer such that 1.ltoreq.j.ltoreq.q, defining one of the zones of the material and having means making it possible to supply appropriate excitation signals on the rows and columns for exciting an optical property of the material.
Numerous means of this type are known for which the excitation is electrical and which e.g. use as the sensitive material a liquid crystal film. The invention more particularly applies to such means, but in more general terms applies to all means incorporating a material, whereof an optical property can be modified with the aid of an electrical excitation. The material can be a solid, liquid, amorphous or crystalline substance. The optical property can be an opacity, refractive index, transparency, absorption, diffusion, diffraction, convergence, rotary power, birefringence, intensity reflected in a given solid angle, etc.
In polychrome matrix screens, each elementary image point is constituted by several of the zones defined by the intersection of the electrodes, e.g. three aligned zones, each covered by a color filter, e.g. respectively red, green and blue. In each of these image points, the intensity of each of these colors can be independently controlled, which makes it possible to obtain by combination all the tones of the visible spectrum,
For example, FIG. 1 shows the structure of a known trichrome or three-color matrix screen. In this screen, each image point X.sub.ij is defined by the intersection of the control row i, constituted by a single electrode 2, and the control column j, constituted by three electrodes 4, 6, 8. These electrodes are transparent and are in each case covered by a color filter which, in the drawing, are respectively red, green and blue for electrodes 4, 6 and 8.
A known control process for such a matrix screen consists, e.g. in the case of a liquid crystal cell where excitation is of an electrical nature, of applying to row i a periodic voltage V.sub.xi of zero mean value and to the other rows a zero voltage, and of simultaneously applying to each electrode of each control column periodic voltages of mean value zero, of the same duration and frequency as the row voltage V.sub.xi, but which are phase-displaced with respect thereto by a quantity .rho..sub.ij. These voltage signals are applied by a row control means 5 for the row electrodes and by a column control means 7 for the column electrodes.
The optical transmission is at a maximum when this phase is 180.degree. and said transmission is zero (extinction) when said phase is 0.degree.. Between these two values, the transmission is attenuated, extinction being greater as the phase approaches 0.degree..
With this structure and this control process, a uniform background, e.g. red, is obtained in the following way. To each red electrode, i.e. each column electrode covered with a red filter, such as electrode 4, is applied by the column control means 7 a voltage signal having a 180.degree. phase displacement with respect to the row signal supplied by the row control means 5 and to the green and blue electrodes is applied by the column control means 7 a voltage signal in phase with the row signal.
In the case of the display of a uniform background, a majority of the column electrodes are addressed with a given phase displacement. This configuration leads to a coupling between the rows and columns affecting the visual quality of the displayed image.
A description will now be given of the origin of the coupling phenomenon with reference to FIG. 2, which shows the signals applied to the row and column electrodes, as well as the parasitic signals produced. In accordance with the conventional control process,, all the row electrodes are connected to ground, with the exception of the row electrode i, in which 1.ltoreq.i.ltoreq.p, to which is applied a voltage square wave, such as signal a of FIG. 2. A periodic voltage is also simultaneously applied to each column electrode. The phase difference between the row signal a applied to row electrode i and the signal applied to a column electrode determines the optical transmission factor of the liquid crystal volume to the right of the intersection of said row electrode and said column electrode.
This phase displacement is of a random nature. However, to simplify the explanation of this coupling phenomenon, it will be assumed hereinafter that all the signals applied to the column electrodes are either in phase or in phase opposition with the row signal a. These signals respectively carry the references b and e in FIG. 2.
As a result of the capacitive effect, a parasitic voltage signal, such as signal c, appears on the row electrodes connected to ground. The origin of this parasitic signal is as follows. At times t between O and T/2, in which T is the period of the voltage signals applied to the electrodes, the voltages on the electrodes are constant and equal to +V or -V. An electric charge -Q appears on a column electrode subject to a voltage +V, to the right of said column electrode and a row electrode i connected to ground, whereas an electric charge +Q appears on said row electrode. On a column electrode subject to a voltage -V, the electric charge produced will be reversed, as will the electric charge appearing on each of the row electrodes.
The algebraic sum .SIGMA.Q of the electric charges present on row i is not generally zero. It is only zero if there are the same number of column electrodes subject to a voltage +V as there are column electrodes subject to a voltage -V. At times t between T/2 and T, symmetrically there will be a charge -.SIGMA.Q on row i.
At T/2, there is consequently a supply of -2.SIGMA. Q electric charges on row i. As the display material is of an insulating nature, these charges are supplied by the row electrode i. Signal c expresses the current constituting this charge transfer. Signal c is in phase with the majority column signal, i.e. it is in phase with signal b if the column signals are preponderantly in phase with signal a and it is in phase with signal e if the majority of the column signals are in phase opposition with signal a. For example, signal c shown in FIG. 2 is in phase with signal b.
As a result of the parasitic capacitive effect, the voltage applied to the material at the intersection of an unselected row and a column to which is applied the signal b is not equal to the latter and is instead equal to signal d representing the difference between signals b and c. In the same way, the material at the intersection of an unselected row and whereof the column signal is represented by signal e is subject to a voltage represented by signal f equal to the difference between signals e and c, instead of being subject to a voltage represented by signal e.
However, it is known that the excitation of the material and consequently its optical transmission varies with the square of the voltage, i.e. with the areas D, F, respectively of signals d, f.
This area difference of the signals d and f, from the visual standpoint, leads to a poor uniformity of the image or picture, as well as to streaks on the screen.
According to the prior art, in order to attempt to limit the coupling effect, it has been necessary to use integrated circuits for controlling the rows of electrodes having a very low access time. This solution causes a certain number of problems, because in order to obtain a low access time, the integrated circuit must have a very low resistance in the conductive state R.sub.on, which requires integrated circuits with a large geometry. Moreover, in order to fully benefit from the limited access times of these control circuits, it is necessary to use very conductive electrodes, whose cost is high, such as In.sub.2 O.sub.3 or Zn.sub.2 O.sub.3 electrodes with a thickness of 125 nm and having a resistance of 40.OMEGA./cm.sup.2.
The aim of the invention is to eliminate the coupling between the control columns and rows of a polychrome matrix screen. This result is obtained on the one hand through the use of an original structure of the screen and on the other hand by a control process such that the algebraic sum of the control signals of the column electrodes is zero at all times.
The elimination of the coupling between the rows and columns makes it possible to reduce the constraints on the access times of the integrated circuits for controlling the rows of the matrix screen. This makes it possible to reduce the geometry of the circuits and use electrodes which are less conductive consequently have a lower cost price.